The use of photoelectric devices or solar cells for the generation of charging currents for use in charging gastight (conventional dry, sealed) batteries in electronic pocket calculators, watches, or the like, is well known. When using such solar cells, it must be remembered, on one hand, that the charging current for the batteries must not exceed a certain limit or the battery will be damaged. On the other hand, in order to maintain proper functioning of the device which is powered by the battery, such as an electronic desk calculator, the current which is furnished by the solar cell should remain independent of the intensity of light which is incident thereon.
The use of a Zener diode connected in parallel to a battery to limit charging current applied to the battery to a predetermined value to avoid damage to the battery by the application of too high a current is well known. Such an arrangement is used when the photocell or solar cell will be exposed to strong light, such as direct sunlight. In a charging circuit for use with an electronic pocket calculator, the employing of a Zener diode in parallel to the battery to be charged results in a substantial disadvantage. When the intensity of light which is falling on the solar cell is substantially less the intensity of direct sunlight, the Zener diode consumes an amount of power which will be sufficient to prevent the battery from being properly charged.